I am not sure, what I am seeing on your images. When I tried to magnify them things became rather blurry.
You may have a severe infestation of armored scale insects. But I cannot tell, because I would need to see a close-up of the splotches on your Opuntia sp.
Have you checked on this page the tab named "Pests and Disease"? Perhaps you can find something that looks like what you are experiencing there.